Could someone explain how this exactly works like in the sentence kaimono ga shitai n desu ga en when you should use it. Thanks.

-n desu ga is the contracted form of "no desu ga". It acts as a sentence softener and is used when the speaker is expecting a response from someone he is speaking to. In the sentence "kaimono ga shitai~n desu ga" (Kaimono ga suru means to go/do shopping) Shitai is the "want to" form of suru, (the -tai is added as an auxiliary adjective to express desire). You could say: Kaimono ga shitai (desu). This means "(I) want to do shopping", but sounds more like an exclamation, which might be necessary if you are making an outright protestation. However, imagine this converation. Issho ni eiga o mimashou ka? Jitsu wa, kaimono ga shitai~n desu ga... When asked if he/she would like to watch a movie, insteade of saying "shitai desu!", they soften their refusal with "~n desu ga". Generally, you should use it in polite speech when declining an offer, or when asking for assistance (like I said it encourages a response from the listener) for example: Toire ni ikitai~n desu ga. Hope this helps.